The basic structure of an OLED device is as follows: one or more organic emission layers are interposed between two electrode layers. The two electrode layers respectively work as an anode and a cathode of the OLED device. The electrodes may be made of a metallic material or a metal oxide material as required. Under the action of an external voltage, electron and hole carriers are respectively injected into the organic emission layer from the directions of the cathode and the anode, and meet each other and are recombined to generate excitons. The energy of the excitons is degraded in the form of light, so that light is radiated, and therefore the electroluminescent effect can be achieved.
In the OLED device, the energy loss mainly exists in two aspects. Firstly, when the carriers are injected into the emission layer for luminescence through recombination, not all the energy can be converted into photons; and one part of energy will loss in the process of radiative transition such as lattice vibration and deep-level impurity transition. This process may be described by the internal quantum efficiency. Secondly, radiated light will be fully reflected at interfaces of anode/substrate, substrate/air and the like and cannot be refracted. Moreover, due to the waveguide mode loss at an anode/organic emission layer interface and the surface plasmon loss and the like near metal electrodes, only about 20 percent of light can transmit through the device and be used for display. This process may be described by the external quantum efficiency.
At present, various methods have been tried to improve the external quantum efficiency, namely improve the light extraction efficiency or the luminous efficiency. For instance, the waveguide mode loss can be reduced by manufacturing a surface microstructure on a metal oxide electrode (e.g., indium tin oxide (ITO)); the total reflection can be reduced by adhering photonic crystals or micro-lens arrays onto a glass substrate; the surface plasmon loss can be reduced by manufacturing a cathode with folds; and an optical micro-cavity structure is utilized.
Although these technologies can greatly improve the luminous efficiency of the device, there are also defects. For instance, as for the methods of forming a periodic or quasi-periodic microstructure pattern on a cathode and adhering the photonic crystals or the micro-lens arrays onto the glass substrate, a nano photo printing technology is always used, and hence the manufacturing process is complex and the difficulty is large. The optical micro-cavity effect is likely to cause the problems such as color-shift of light and narrower viewing angle.